The main way I tend to make money if I'm feeling on the straight and narrow (that goes out the window when I want to get serious about breeding), is to run a "stud farm". Start a new game, immediately put all your horses up for auction, apart from one stallion. That should net you a decent amount of cash - it varies, but usually you'll end up with between 600k to >1mil. With that money, I buy the breeding barn, then head to the auctions and pick as many super cheap (10k and under) mares as I can. Retire all of these to the breeding barn - stallion included - and start breeding. You'll need to check back every couple of weeks to send another mare to your stallion (it takes 14 days for breeding to take place for some reason). Usually in year 1 I try and end up with at least 10 mares.
When a new game year starts, the mares have given birth, and you'll have a tab in the breeding section of your barn for yearlings. Open up each yearling's page individually and send them all to auction. This should net you at least a million. Rinse and repeat for the next few years. As you build up money you can start going to the breeding auctions (the blue-green circle on the sales tab indicates one of these auctions) and pick up group winning mares to breed to the best outside studs. These foals are much more likely to be decent than the ones you've been breeding so far, so I tend to keep these and race them for myself.
Having said all of that, the breeding in the game may not be working properly at the moment - the offspring of g1 winning horses tends to be just as crap as that of claimers - so buying g1 winning mares is probably pointless at the moment. Especially because future updates will likely render our current saves unplayable. But this remains a legitimate and viable way to make some early game money, and it can be interesting to track the success of the foals you've bred and sold.
For selecting jockeys: I just tend to pick the most expensive one that's not above my horse's carry weight. I'm not sure how much impact your choice of jockey makes.
Preferred ground: I can't see any indicator as to the condition of the track when entering races at the moment. In SO6 there was a small icon next to each meeting in the declarations screen that gave a weather forecast. That may have just been overlooked in SO7 at this point, or I may be blind and it is in fact there.
Horses are usually good to run every three or so weeks, though this varies from horse to horse - some can take a lot more racing than others. I check the head lad's comments and if he says they're raring to go or something similar, then enough time has passed for them to race again. I make a mental note of how long it takes for them to be ready to after their last race and enter them at that time interval (if it took three weeks, you can safely enter them every three weeks). However, 2 and 3yo horses are particularly sensitive to being overraced, so keep checking the head lad's comments. He'll say something like "Care! You are close to overracing this horse." when they've about had enough. At that point I think they're usually done for the season.
Oof. That was a novel
Hope it helped some.